It is well known that metallurgical silicon can be obtained by heating, in a high-temperature environment, silica (quartz) and the element carbon extracted from coal, petroleum or the like. However, the purity of such metallurgical silicon is approximately 99% (2N). Thus, it is normally used as the starting material for further purification and refinement such that it reaches a purity of 99.9999% (6N)˜99.999999999% (11N), suitable for use in solar cells as high-purity silicon, which is often difficult to achieve.
Traditional purifying/refining methods can be roughly divided into two types: one type typified by the commonly called “Siemens” method and the other typified by the Upgraded Metallurgical-Grade (UMG) silicon production process. For the Siemens method, the quality of the resulting product is good and the process is well established, but it has the disadvantages of high production cost and requiring the use or production of poisonous materials.
As for UMG silicon, although the purity is lower than in the Siemens method, the cost is lower and the process does not produce or use poisonous materials. Traditional UMG silicon production processes use the following techniques for providing inexpensive silicon:
1. Slag formation;
2. One-directional cooling;
3. Vacuum vaporization; or
4. A combination of the above.
Using the above techniques and metallurgical silicon with a purity of 2N as the starting material, the silicon product can reach a purity of 6N, which is still not pure enough for solar cell applications.
Therefore, UMG silicon (UMG-Si) with a purity of 6N is then used as the raw material for further purification using single-crystal and/or ingot casting methods to obtain ingots in a shape suitable for solar cell applications.
However, when examining solar cells produced using UMG-Si as the raw material, it is found that its electromotive force (EMF) varies (decreases) over time in actual usage.
Thus, silicon produced using metallurgical methods is inexpensive but has the shortcoming that its EMF changes over time.
To overcome this problem, UMG-Si and high-purity silicon (e.g., made by the Siemens method) are combined to lessen the time-varying EMF problem. Yet, such an approach does not solve the root cause but rather is an expedient measure.
Despite using Floating Zone (FZ) or Czochralski (CZ) methods on UMG-Si to increase its purity, oxygen contained in the silicon will have an adverse effect on the EMF. In addition, when the amount of oxygen contained is large, crystalline defects will occur even if the amount of impurities (e.g., carbon, iron, copper, and nickel) contained is low. As a result, the EMF of the silicon product will be decreased over time.
Therefore, excess oxygen contained in traditional UMG-Si is the main cause for crystalline defects, and should be reduced. Meanwhile, other impurities contained in the silicon must also be minimized for purification/refinement purposes.